Method of manufacturing electrical resistance elements and elements manufactured by the method

ABSTRACT

An electric resistance element of molybdenum disilicide has a thin incandescent zone whose dimensions are formed by chemical decomposition, dissolution or corrosion.

United States Patent [72] Inventor Nils Gustav SchreweliusHallstahammer, Sweden [2i Appl. No. 677,569 [22] Filed Oct. 24, 1967[45] Patented Sept.2l,l97l [73] Assignee Alrtlebolaget KanthalHallstahlmmar, Sweden [32] Priority Oct. 24, 1966 [33] Sweden [31]14564-66 [54] METHOD OF MANUFACTURING ELECTRICAL RESISTANCE ELEMENTS ANDELEMENTS MANUFACTURED BY THE METHOD 10 Claims, 2 Drawing Figs.

52 u.s.c| 156/6, 15612191 1i,219/s 3,3 31357, 13/25 [5 l] Int. Cl ..C23f17/00, H0lj 1/02 [50] Field of Search l56/2, 6, 8,ll,l8;29/6ll;2l9/553;3l3/357;13/25 [56] References Cited UNITED STATESPATENTS 2,432,800 l2/l947 Reichold 156/8 X 3,174,920 3/1965 Post OTHERREFERENCES Chemical Milling for Thin Complex Parts. Metal industry 8Aug. 1963 Vol. 103 P. I88

Primary ExaminerJacob H. Steinberg Atromey Fidler, Bradley, Patnaude &Lazo ABSTRACT: An electric resistance element of molybdenum disilicidehas a thin incandescent zone whose dimensions are formed by chemicaldecomposition, dissolution or corrosion.

METHOD OF MANUFACTURING ELECTRICAL RESISTANCE ELEMENTS AND ELEMENTSMANUFACTURED BY THE METHOD The present invention relates to a method ofmanufacturing electrical resistance elements substantially consisting ofmolybdenum disilicide and having an incandescent zone and at least onethicker terminal. Such electrical resistance elements have been knownfor a long time.

The natural brittleness of the material in cold condition has limitedthe use of molybdenum disilicide substantially to elements having arather large cross sectional area. As an example of such elements theproven type of element sold under the registered trade mark of KANTHALSUPER may be mentioned, said element consisting of an incandescentportion in U-shape and having straight terminals. The incandescentportion may have a diameter of about 6 mm. an the terminals which aregenerally connected to the incandescent portion by resistance buttwelding, may have a diameter of about 12 mm. ln these thick terminalsthe temperature will be low even though the incandescent portion has atemperature of about l,600 to l,700 C., and thereby it becomes possibleto con nect them with contacts of aluminum, for instance.

When trying to make resistance elements of the type mentioned but havingconsiderably smaller dimensions, such as diameters of the magnitude 1mm. for the incandescent portion, it has in practice been found verydifficult to weld on the terminals as the material is hard to handleowing to its great brittleness. Therefore, in the case of thin elementsof molybdenum disilicide it has been necessary to choose another way,viz, not to use a welded-on thicker terminal. Instead, the whole elementis designed as an incandescent portion soldered to air or liquid cooledcontacts. Owing to the strongly positive temperature coefficient of theelement material for the electrical resistance, it may be possible toobtain rather cold contacts in such elements.

- These known thin elements which are soldered to contacts have,however, proved troublesome in use. The majority of metallic materials,with the exception of gold and silver, react with the element materialat high temperatures, and because of the difierences in heat expansioncoefficient the soldered joints do not become durable, particularly ifthe elements are used intermittently which, as a rule, is the case.

The object of the present invention is to remedy the above difficultiesin a simple and cheap way, and a thin molybdenum disilicide element withat least one thicker terminal is characterized, according to theinvention, substantially in that at least the incandescent portion ofthe element has been given its final dimension by removing certainportions of an element blank by a chemical removal process such aschemical dissolution, decomposing or corrosion.

The simplest way of carrying out the method according to the inventionis to submerge a U-shaped rod blank in an acid mixture for such a longperiod of time and to such depth that the acid corrodes away the amountof material required to give the incandescent portion the desired finaldiameter and length.

The chemically dissoluting, decomposing or corroding agent which hasproven most useful in this connection is an acid mixture containinghydrofluoric acid, nitric acid and, if desired, water.

Resistance elements which are in the known way built up of a powdermetallurgical composition of molybdenum disilicide and a glass rich inSiO are dissolved in said acid mixture as the molybdenum disilicide andthe glass component are dissolved at approximately the same rate.Thereby a smooth surface and a controllable cross sectional area of theportion treated with acid are obtained. However, resistance elementssubstantially consisting of molybdenum disilicide may also contain othersubstances as additions in order to modify different properties.

. An example of such an addition is finely powdered silicon car bidewhich is then substituted for the glass component in the above example.The glass as well as the silicon carbide prevents the grain growth ofthe molybdenum disilicide at high temperature and also gives thematerial other advantageous properties, such as plastic workability inheat, for instance. However, the silicon carbide component in such anelement is not dissolved at all or only dissolved very slowly in theacid mixture mentioned, but nevertheless it is possible according to theinvention to produce elements with thin incandescent portions bychemical treatment provided that the molybdenum disilicide is the maincomponent; unattacked silicon carbide present in rather insignificantamounts will automatically fall off from the goods dissolved. Also otheradditions and modifications of the composition may be permittedaccording to the invention, such as a modification of the MoSi,component by adding as alloy components small amounts of other metalswhich do not influence the chemical solubility.

The interface between air and liquid in an acid bath for chemicaldissolution as per above has proved to have a disadvantageous influenceon the smoothness of the corroded portion. Therefore, it has provedconvenient to keep the element in motion during the treatment to avoid areduced diameter of the portion located at the interface. Another way ofavoiding the problems with the interface between air and liquid is tocoat those portions of the element blank which are not to be corrodedwith a layer of an acidproof material, such as plastics, the elementblank being submerged in the acid bath to such a depth that theinterface air-liquid will be within the plastics-coated area.

Thus, the method according to the invention presupposes that there is nointercrystalline corrosion or any other type of attack interiorly of theelement. Therefore, it is important that the bodies containingmolybdenum disilicide are free of pores.

The porosity should be less than 2 percent, preferably below 0.5-1percent. To obtain such low porosity it is convenient to use low meltingglass as binding agent for the molybdenum disilicide.

The invention is illustrated by the following practical example:

A straight homogenous rod of 4.6 mm. diameter and having a porosity lessthan 1 percent consisting of a fine-grained powder metallurgical densebody containing 90 percent by weight of MoSi and 10 percent by weight ofa glass rich in SiO,, which has been produced by pressing and sinteringMoSi and bentonite was bent in heat into U-shape, the legs having alength of 50 mm., and the space between the legs being 15 mm. ThisU-shaped blank was half submerged in a cold mixture of 50 parts ofvolume of concentrated nitric acid, 50 parts by volume of concentratedhydrofluoric acid and l0 parts by volume of water. The element was keptsubmerged in this mixture for 2 minutes, while being slightly moved, andwas then carefully rinsed in water. After the treatment, an incandescentzone having the diameter 2.3 mm., had been formed. The element wasprovided with sprayed-on contact layers of aluminum, whereupon it wasconnected in a current circuit and permitted operating temperatures of1,600 C. and more in the incandescent zone, while the thicker,uncorroded terminals had a temperature of less than 200 C. in thecontact layers.

By the new method it is possible to produce miniature elements in aneconomical and practical way. This method is useful for the manufactureof very small elements intended for gas igniters, for instance, andhaving a smaller diameter than 1 mm. for the incandescent zone, such as0.4 mm., and terminals of 1 to 2 mm. The advantage of the methodaccording to the invention is, inter alia, that it is possible toaccurately control the diameter and that the difficult bending in heatof very thin incandescent zones may be dispensed with. Thus. it

is possible to shape a small element having a leg length of 50 mm., forinstance, and a space between the legs of only 3 mm., by starting from astraight rod with adiameter of 1.5 mm.,

. bending it in heat into a U with the desired leg space and then iseven possible to make elements with incandescent zones having diametersof 0.1 to 0.2 mm.

As it is not practicable to make wires of molybdenum disilicide bydrawing, the new method offers possibilities of making such thindimensions as could not previously be manufactured with the desirableaccuracy of measures.

Also, the method offers, possibilities of automatizing the production,as it is easy to make such arrangements that the resistance of theelement blank may be caused, during the acid treatment, to control themoment at which the blank is removed from the acid bath.

The method may also be applied such that a blank of molybdenumdisilicide in the form of a tube or a plate, for instance,

is coated with a material resistant to the acid mixture, such asplastics, for instance, and that a desired pattern is drawn on thesurface so that a selective dissolution of the material can then takeplace in accordance with particular wishes. For instance, it is possiblein this way to make from a very thin plate a flat resistance coil withbroader terminals. Such flat elements may be used in cigarette lighters,for instance.

Two embodiments of elements according to the invention are shown in theaccompanying drawing wherein FIG. 1 is a perspective view of ahairpin-shaped element and FIG. 2 is a plan view of an element formed asa flat spiral. The elements in FIG. 1 which consists essentially of MoSihas two terminals 11, 12 with flame sprayed aluminiumcontacts 11A, 12Aand of a diameter of about 1 mm. and a length of about 60 mm. each. Theterminals 11, 12 are parallel to each other and spaced apart about 3-10mm. The bent incandescent zone 13 which is formed from the samerodshaped blank as the terminals 11, 12 has had its diameter reduceddown to about 0.4 mm. by chemical corrosion as discussed above.

The element 14 in FIG. 2 has been formed with an incandescent zone 15and two wider terminal portions 16, 17 by corroding away a spiral shapedportion to form a spiral slot 18 in a flat, plate-shaped blank,essentially of MoSi in the above-described manner.

A plurality of modifications and alterations of details are possiblewithin the scope of the invention idea.

I claim: 1. A method of manufacturing an electrical resistance elementhaving a thin incandescent zone, comprising the steps of forming a blankof said resistance element from molybdenum disilicide and a glass richin silicon dioxide to have a porosity lower than 2 percent and having atleast one electrode portion conforming substantially to the electrodeportion of said resistance element and having an incandescent zonethicker than the incandescent zone of said electrical resistanceelement, I

bringing said incandescent portion of said blank into contact with achemical agent capable of removing molybdenum disilicide for asufficient time to uniformly thin said incandescent portion while saidelectrode is protected from said chemical agent; and

separating said incandescent portion from said chemical agent beforesufficient material has been removed from said incandescent portion toform any aperture in said incandescent portion, whereby the desireddimensions ofcomprising nitric acid and hydrofluoric acid into contactwith said incandescent portion.

3. A method in accordance with claim 2 in which said step of bringing achemical agent into contact with said incandescent portion comprises thestep of bringing an acid mixture consisting essentially of 50 parts byvolume of concentrated nitric acid and 50 parts by volume ofconcentrated hydrofluoric acid into contact with said incandescentortion.

4. A method in accordance with claim in which said step of bringing saidchemical agent into contact with said incandescent portion comprises thestep of submerging said incandescent portion into said chemical agentwhile maintaining said electrode portion free from said chemical agent.

5. A method in accordance with claim 4 in which said step of formingsaid electrical resistance blank comprises the step of coating saidelectrode portion with a masking substance that is not affected by saidchemical agent, whereby the interface between the chemical agent and theatmosphere is prevented from creating a discontinuity in said electricalresistance element.

6. A method in accordance with claim 4 in which said electricalresistance blank is moved with respect to said chemical agent whilesubmerged therein, whereby said interface between said chemical agentand the atmosphere is prevented from causing a discontinuity in saidelectrical resistance element.

7. A method according to claim 1 in which said step of forming saidelectrical resistance blank includes the step of forming said electricalresistance blank from a material consisting of at least 60 percent byvolume of molybdenum disilicide and at most 40 percent by volume ofceramic additions.

8. A method in accordance with claim 1 in which:

said step of forming said electrical resistance blank includes the stepof forming a straight rod of approximately 5 millimeters in diameterwith a porosity less than 1 percent of a fine-grained powdermetallurgical body containing percent by weight of molybdenum disilicideand 10 percent by weight of a glass rich in silicone dioxide which hasbeen produced by sintering molybdenum disilicide and bentonite and thestep of bending said rod while heated into a U-shape with the legs ofthe U having a length of approximately 50 millimeters and with a spacebetween the legs of approximately 15 millimeters; said step of bringinga chemical agent into contact with said incandescent portion includesthe step of submerging the U-shaped blank half way into a cold mixtureof 50'parts by volume of concentrated nitric acid, 50 parts by volume ofconcentrated hydrofluoric acid and [0 parts by volume of water. forapproximately two minutes while being moved; and a said step ofseparating said'incandescent portion and said chemical agent includesthe steps of removing said U- shaped blank from said cold mixture andrinsing with water. Q 1 9. An electrical resistance elementformed inaccordance with the method of claim 1 and having a rodlike cross sectionin which the diameter of the incandescent zone is less than 6millimeters.

10. An electrical resistance element formed in accordance with themethod of claim 1 and having a rodlike cross section in which thediameter of the incandescent zone is less than i millimeter and thediameter of the electrode portion is more than 1 millimeter.

2. A method in accordance with claim 1 in which the step of bringing the chemical agent into contact with said incandescent portion comprises the step of bringing an acid mixture comprising nitric acid and hydrofluoric acid into contact with said incandescent portion.
 3. A method in accordance with claim 2 in which said step of bringing a chemical agent into contact with said incandescent portion comprises the step of bringing an acid mixture consisting essentially of 50 parts by volume of concentrated nitric acid and 50 parts by volume of concentrated hydrofluoric acid into contact with said incandescent portion.
 4. A method in accordance with claim 1 in which said step of bringing said chemical agent into contact with said incandescent portion comprises the step of submerging said incandescent portion into said chemical agent while maintaining said electrode portion free from said chemical agent.
 5. A method in accordance with claim 4 in which said step of forming said electrical resistance blank comprises the step of coating said electrode portion with a masking substance that is not affected by said chemical agent, whereby the interface between the chemical agent and the atmosphere is prevented from creating a discontinuity in said electrical resistance element.
 6. A method in accordance with claim 4 in which said electrical resistance blank is moved with respect to said chemical agent while submerged therein, whereby said interface between said chemical agent and the atmosphere is prevented from causing a discontinuity in said electrical resistance element.
 7. A method according to claim 1 in which said step of forming said electrical resistance blank includes the step of forming said electrical resistance blank from a material consisting of at least 60 percent by volume of molybdenum disilicide and at most 40 percent by volume of ceramic additions.
 8. A method in accordance with claim 1 in which: said step of forming said electrical resistance blank includes the step of forming a straight rod of approximately 5 millimeters in diameter with a porosity less than 1 percent of a fine-grained powder metallurgical body containing 90 percent by weight of molybdenum disilicide and 10 percent by weight of a glass rich in silicone dioxide which has been produced by sintering molybdenum disilicide and bentonite and the step of bending said rod while heated into a U-shape with the legs of the U having a length of approximately 50 millimeters and with a space between the legs of approximately 15 millimeters; said step of bringing a chemical agent into contact with said incandescent portion includes the step of submerging the U-shaped blank half way into a cold mixture of 50 parts by volume of concentrated nitric acid, 50 parts by volume of concentrated hydrofluoric acid and 10 parts by volume of water for approximately two minutes while being moved; and said step of separating said incandescent portion and said chemical agent includes the steps of removing said U-shaped blank from said cold mixture and rinsing with water.
 9. An electrical resistance element formed in accOrdance with the method of claim 1 and having a rodlike cross section in which the diameter of the incandescent zone is less than 6 millimeters.
 10. An electrical resistance element formed in accordance with the method of claim 1 and having a rodlike cross section in which the diameter of the incandescent zone is less than 1 millimeter and the diameter of the electrode portion is more than 1 millimeter. 